Flat panel displays are widely used in a variety of applications, including computer displays. One type of device suited for such applications is the field emission display. Field emission displays typically include a generally planar substrate having an array of projecting emitters. In many cases, the emitters are conical projections integral to the substrate. Typically, the emitters are grouped into emitter sets where the bases of the emitters in the emitter sets are commonly connected. A conductive extraction grid is positioned above the emitters and driven with a voltage of about 30 V-120 V. The emitter sets are then selectively activated by connecting the emitter sets to ground. Grounding the emitter sets creates an electric field between the emitters and the extraction grid of any intensity that is sufficient to extract electrons from the emitters and it also provides a current path between the emitters and ground.
The field emission display also includes a display screen mounted adjacent the substrates. The display screen is formed by a glass plate coated with a transparent conductive material to form an anode biased to about 1-2 kV. A cathodoluminescent layer covers the exposed surface of the anode. The emitted electrons are attracted by the anode, and they strike the cathodoluminescent layer causing the cathodoluminescent layer to emit light at the impact site. The emitted light then passes through the glass plate and the anode where it is visible to a viewer.
The brightness of the light produced in response to the emitted electrons depends, in part, upon the rate at which the electrons strike the cathodoluminescent layer, which in mm depends upon the magnitude of the emitter current. The brightness of each area can thus be controlled by controlling the current flow to the respective emitter set. By selectively controlling the current flow to the emitter sets, the light from each area of the display can be controlled and an image can be produced. The light emitted from each of the areas thus becomes all or part of a picture element or "pixel."
One problem in such field emission displays is spreading of the electrons as they are emitted from the emitters. When the emitters emit electrons, not all of the electrons travel directly toward the anode. Instead, the electrons may spread out as they travel toward the anode. As a result, when the emitter set is activated, the area of the cathodoluminescent layer struck by the electrons may be larger than the desired size of the pixel. Consequently, the light emitted from the area may "bleed" into an adjacent pixel, causing loss of resolution and picture quality.
Additionally, the number of electrons emitted from the emitter may sometimes be insufficient to produce sufficient brightness of the pixel. Various techniques have been applied to improve the efficiency of electron emission from the emitters. For example, emitters have been coated with a material having a low work function to increase the emission of electrons from the emitters. However, to the inventor's knowledge, no attempts have been made to provide a gain element in the path between the emitters and the anode to increase the number of electrons striking the cathodoluminescent layer.